Add initial implementation of meta

[Hoolean]

Hello!

This PR adds adds read and write for the meta table, including a codegen definition, some manual parts to handle the offset slices, and plumbing to surface the table within read-fonts and write-fonts.

This is my first contribution, so where possible I've borrowed liberally from the implementation of other tables, especially name (for its handling of offset prescribed-length data).

This is an early implementation, and has not had the manual and automated testing it will require yet. Additionally, there is an implementation of the FontWrite trait for &[u8] that may be redundant.

As such, I'm making the PR as a draft for now, until I can gather feedback and dig a bit deeper :)

_{Note: this is a personal contribution independent of my employer, and so I've submitted from a fork under my personal profile and email to make this distinction}

[cmyr]

Nice, I think this is the right general direction, but after staring at it for a while I think it makes sense for us to use more types to describe the actual metadata contents; and since figuring out how best to do that requires more or less just writing the implementation I took the liberty of doing that in #1244.

Overall though I think this was a reasonable first pass. If you're happy with #1244 we can merge that, and then as future work we can actually implement some types that enforce the spec for the LangScriptTag types.

Oh: we also need some test data, that's missing from #1244 and it's complicated enough to need it.